My son is 6 and will join me in the shop anytime. I willed him my old toolchest and at every swap meet he gets a new tool or something that he will have forever.
He definitely has his favorite car (my 67 Malibu), but I encouraged him to wait until he's ridden in the L78 car. If he gets tired of helping he usually asks to sit in his go kart and practice the gas and brake pedals.
We even went so far as to paint two walls of his room orange and put a white Yenko Duece stripe on it.
My daughter is 4 and she's not the gearhead my son is. She loves going to the go kart races and is certain that as soon as she's old enough she will drive it.
I have one more coming in May, so I've warned my wife that we'll have to get at least one more car, just to keep the kids even
It's a great way to spend some good time with your kids and have them learn something they can use forever, including a work ethic.
Blair